allow disabling MENU_BUTTON_BACK during ScreenStage

stepmania/src/ScreenStage.cpp

@@ -30,6 +30,8 @@ void ScreenStage::Init()
 {
 	Screen::Init();
 
+	ALLOW_BACK.Load( m_sName, "AllowBack" );
+
 	m_bZeroDeltaOnNextUpdate = false;
 
 	SOUND->StopMusic();
@@ -148,6 +150,9 @@ void ScreenStage::MenuBack( PlayerNumber pn )
 	if( m_In.IsTransitioning() || m_Out.IsTransitioning() || m_Back.IsTransitioning() )
 		return;
 
+	if( !ALLOW_BACK.GetValue() )
+		return;
+	
 	this->ClearMessageQueue();
 	m_Back.StartTransitioning( SM_GoToPrevScreen );
 	SOUND->PlayOnce( THEME->GetPathS("Common","back") );

stepmania/src/ScreenStage.h

@@ -9,6 +9,7 @@
 #include "Character.h"
 #include "BitmapText.h"
 #include "ActorUtil.h"
+#include "ThemeMetric.h"
 
 class ScreenStage : public Screen
 {
@@ -21,6 +22,8 @@ public:
 	virtual void MenuBack( PlayerNumber pn );
 
 private:
+	ThemeMetric<bool>	ALLOW_BACK;
+
 	Transition		m_In, m_Out, m_Back;
 	AutoActor		m_Overlay; // overlays all elements except bitmaptexts
 	BitmapText		m_SongTitle;